Where can I purchase a VPN?

What are you attempting to achieve?

Some WiFi Routers have the capability of running a VPN Gateway - to which other VPN clients can connect. Similarly, some have a built-in VPN client that can be used to connect to a corporate VPN Gateway, or a commercial VPN service. As for your iPad, iPadOS has a built-in VPN client that can be configured to connect with VPN Gateway services as needed.

While VPNs have their place within a security solution, they do not necessarily provide end-to-end security. That said, they are very useful in protecting network traffic from interference or interception over an untrusted network segment - such as using a public (open) WiFi network.

If you are able to describe precisely what you hope to achieve in using a VPN, contributors will be better able to provide appropriate qualified advice as to how you might configure your devices to meet the security target.

Routers don't have VPNs, there may be some in the App Store for your iPod although they aren't recommended. Why do you think you need one?